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The photographic news
- Bandzählung
- 27.1883
- Erscheinungsdatum
- 1883
- Sprache
- Englisch
- Signatur
- F 135
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- Hochschule für Grafik und Buchkunst Leipzig
- Digitalisat
- Hochschule für Grafik und Buchkunst Leipzig
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- Bandzählung
- No. 1273, January 26, 1883
- Digitalisat
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Zeitschrift
The photographic news
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Band
Band 27.1883
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- Titelblatt Titelblatt I
- Register Index III
- Ausgabe Ausgabe 1
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- Ausgabe Ausgabe 641
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- Ausgabe Ausgabe 801
- Ausgabe Ausgabe 817
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Band
Band 27.1883
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- Titel
- The photographic news
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60 THE PHOTOGRAPHIC NEWS. [JANUARY 26, 1883. be given; with the latter, from thirty to sixty seconds will suffice, unless the proportion of the restraining acid be increased to an unnecessary extent. A good tone is obtained by the following developer :— Pyrogallic acid 100 grains Citric acid 60 „ Acetic acid ... ... ... ... ... 2 ounces Water 20 „ After this is applied, a short time will elapse ere the image appear, after which it will rapidly gain strength. The mistake into which the inexperienced most usually fall, is to carry the development too far, by which a deep, heavy-looking imudgy picture results in the transfer, although when viewed as a transparency on the glass it may seem all right. Experience only can guide one in this matter, and fortunately it is an experi ence that may be gained in course of an hour’s active work. A twelve-grain solution of protosulphate of iron would develop the picture with a far greater degree of rapidity than the developer already given, and with an exposure very much shorter, but the tone would be unpleasant. Citric acid ensures dark tones, but to enable it to exercise this influence upon the image that is being formed, it is necessary that, to counteract its retard ing power, the strength of the iron be increased to a considerable extent, and also that the exposure be somewhat prolonged. Cyanide of potassium must not be had recourse to for fixing, on account of its tendency to give a light colour to the deposited silver. The proper fixing agent is a saturated solution of hypo sulphite of soda. But to recover an overdone picture, when it is inexpedient to make a second trial, cyanide is very serviceable. It should be allowed to act upon the image until the high-lights are seen to be denuded of the silver by which they were obscured. Should the tone have been lightened to too great an extent by this treatment, it may be darkened by the application of a weak solution of the chlorides of either platinum, gold, or mercury. It is worthy of remark that a wash of the mercury salt, when allowed to act no farther than to blacken the image, yields a picture which may be considered as tolerably permanent. The image having been fixed and washed, a sheet of transfer paper, previously soaked in cold water for a few minutes, until it has a slightly slimy feeling, is laid face down upon the collo dion picture, pressed into contact with it, and placed away to dry. After a few hours it may be raised at one corner and stripped away from the glass, carrying with it the collodion pic ture. The “ transfer ” has now an exceedingly glossy surface, and when mounted on a card by means of thin glue, is ready for receiving oil colours without any preparation, should it be desired to finish it in that manner. If a matt surface be desired, the transfer should be stripped from the glass before it is quite dry. The transfer paper for this process is made by placing four ounces of gelatine in a quart of water, allowing it to soak for half an hour, and then placing the vessel containing it into warm water to liquefy the swollen gelatine. Four grains of chrome alum, previously dissolved in a little warm water, are now added and incorporated with the gelatine. Good stiff paper, similar to a fair sample of heavy writing-paper, having been previously cut into sheets the required size, is floated sheet by sheet on the surface of the gelatine, which is kept warm by a water bath. Sponging over the paper with the gelatine answers equally as well as floating it. If the transfer does not strip away properly from the plate, it is caused by one or the other of the followingThe glass was dirty, and not sufficiently prepared with French chalk, or the coating of the transfer paper with gelatine has been too thin. In either case the remedy is obvious. The two principal forms of solar camera are described in the following words:— The form of solar camera most commonly preferred is that introduced by Woodward, which is shown in the adjoining diagram, in which solar rays, r r, fall upon a mirror, A B, and are reflected upon the condenser, by which they are made to converge to a point f, passing through in their course, first, negative J, and then the objective L. They are projected for ward to a focus on a screen in front, on which is fastened by pins the sensitive paper. The wood-work of the camera is repre sented by E F G H, and certain fittings by K D. The size of the condenser varies from nine to twenty inches; the larger the lens is the more light it collects, and consequently the more quickly is the positive printed. For instance, if a condenser of nine inches is employed, and with it an exposure of half an hour is required to obtain an enlargement by direct printing on silver- chlorized paper, a condenser of double the area will print the picture in half the time. The focal length of the condenser should not be less than twice its diameter, nor more than three times. If a small condenser is used, sufficient light may not be collected to impress the image within a reasonable time. On the other hand, if a large condenser is used, the errors arising from spherical aberration become considerable. The objective, or lens L, may be an ordinary portrait com bination ; but care must be taken, in this case, that the lens which faces the ground glass, as in ordinary work, now faces the negative to be reproduced, J. The management of the apparatus is very simple. It is suffi cient to place the part E H B A of the solar camera in an open ing in a darkened window, to communicate by means of the adjusting screws, B and D, the movements necessary for keeping the solar rays always reflected in the same direction, I f; to properly adjust the negative, J, so that its enlarged image is sharply formed on a screen, placed at a distance and perpendicu lar to the optical axis of the apparatus ; and, lastly, to substitute for the screen a sheet of sensitive paper or other photographic surface. Considerable improvements have been made in the solar camera of Woodward, notably those by Dr. Monckhoven, who, to cure the evils arising from the aberrations of sphericity, whereby the various transmitted rays do not arrive at the same focal point, constructed his condenser on what is called the dialytic principle. This principle consists in intercepting the cone of rays from a single crown-glass condensing lens by a con cave lens, by which the aberrations, both of sphericity and colour, may be the former considerably, and the latter altogether, remedied. The accompanying sectional diagram of this earners will serve to show its construction. The large condenser, A B, is what is called a crossed lens, one side being more convex than the other. The more convex side is placed towards the source of light. The refracted rays are intercepted in their passage through the camera by the corrrect’ ing lens, C D, which is concavo-convex, the concave side facing the condenser. This lens simply corrects for spherical and chro matic aberrations. The wooden frame, E F, which holds the negative to be enlarged, is movable by means of a milled-head screw, G, nearer or farther away from the lens, M, when required. The front of the camera to which the objective or enlargi2s lens is fixed is also movable towards or from the frame, EF,b means of the screw, L. Gorxespondence. PAPER NEGATIVES. r Sir,—Hearing of so many who would use bromide PaPGt instead of plates, both for portability and cheapness, b
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